Cement compositions with improved mechanical properties and methods of cementing in subterranean formations

ABSTRACT

Cement compositions with improved mechanical properties and associated methods are provided, which are useful in conjunction with subterranean well operations. In certain embodiments, the cement compositions comprise carbon fibers, rubber particles, a hydraulic cement material, sufficient water to form a pumpable slurry, and optionally other ingredients including a dispersant, a weighting agent, a retarding or accelerating agent, or the like.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional patent application of commonly-ownedU.S. patent application Ser. No. 10/374,296, filed Feb. 25, 2003,entitled “Cement Compositions with Improved Mechanical Properties andMethods of Cementing in Subterranean Formations,” by Lance E. Brothers,et al., which is incorporated by reference herein for all purposes.

BACKGROUND

The present invention relates to subterranean well cementing operations,and more particularly, to cement compositions having improved mechanicalproperties and associated methods.

Hydraulic cement compositions are commonly utilized in subterranean wellcompletion and remedial operations. For example, hydraulic cementcompositions are used in primary cementing operations whereby strings ofpipe such as casing and liners are cemented in well bores. In performingprimary cementing, a hydraulic cement composition is pumped into theannular space between the walls of a well bore and the exterior surfaceof the pipe string disposed therein. The cement composition is permittedto set in the annular space, thereby forming an annular sheath ofhardened substantially impermeable cement therein that substantiallysupports and positions the pipe string in the well bore and bonds theexterior surfaces of the pipe string to the walls of the well bore.Hydraulic cement compositions also are used in remedial cementingoperations such as plugging highly permeable zones or fractures in wellbores, plugging cracks in holes in pipe strings, and the like.

Set cement in wells, and particularly the set cement sheath in theannulus of a well, may fail due to, inter alia, shear and compressionalstresses exerted on the set cement. This may be particularly problematicin high temperature wells, which are wells wherein fluids injected intothe wells, or produced from the wells by way of the well bore, causes atemperature increase over initial cement setting conditions of at leastabout 100° F. In these types of wells, set cements often fail as aresult of the stresses exerted on the set cement.

The stress exerted on the cement as referred to herein means the forceapplied over an area resulting from the strain caused by the incrementalchange of a body's length or volume. The stress is generally thought tobe related to strain by a proportionality constant known as Young'sModulus. Young's Modulus is known to characterize the flexibility of amaterial. In a well bore sealing application, the Young's Modulus fornonfoamed cements is about 3×10⁶ lb_(f)/in², and for steel casings, theYoung's Modulus is about 30×10⁶ lb_(f)/in².

There are several stressful conditions that have been associated withwell bore cement failures. One example of such a condition results fromthe relatively high fluid pressures and/or temperatures inside of theset casing during testing, perforation, fluid injection, or fluidproduction. If the pressure and/or temperature inside the pipeincreases, the resultant internal pressure expands the pipe, bothradially and longitudinally. This expansion places stress on the cementsurrounding the casing causing it to crack, or the bond between theoutside surface of the pipe and the cement sheath to fail in the formof, inter alia, loss of hydraulic seal. Another example of such astressful condition is where the fluids trapped in a cement sheaththermally expand causing high pressures within the sheath itself. Thiscondition often occurs as a result of high temperature differentialscreated during production or injection of high temperature fluidsthrough the well bore, e.g., wells subjected to steam recovery processesor the production of hot formation fluids. Other stressful conditionsthat can lead to cement failures include the forces exerted by shifts inthe subterranean formations surrounding the well bore or otherover-burdened pressures.

Failure of cement within the well bore can result in radial orcircumferential cracking of the cement as well as a breakdown of thebonds between the cement and the pipe or between the cement sheath andthe surrounding subterranean formations. Such failures can result in atleast lost production, environmental pollution, hazardous rigoperations, and/or hazardous production operations. A common result isthe undesirable presence of pressure at the well head in the form oftrapped gas between casing strings. Additionally, cement failures can beparticularly problematic in multi-lateral wells, which include verticalor deviated (including horizontal) principal well bores having one ormore ancillary, laterally extending well bores connected thereto.

In both conventional single bore wells and multi-lateral wells havingseveral bores, the cement composition utilized for cementing casing orliners in the well bores must develop high bond strength after settingand also have sufficient resiliency, e.g., elasticity and ductility, toresist loss of pipe or formation bonding, cracking and/or shattering asa result of all of the stressful conditions that may plague the well,including impacts and/or shocks generated by drilling and other welloperations.

SUMMARY

The present invention provides a cement composition having improvedmechanical properties including tensile strength and elasticity, andassociated methods.

One embodiment of the cement compositions of the present inventioncomprises a hydraulic cement; water; rubber particles; and carbon fiberswherein the carbon fibers have a mean length of about 1 mm or less.

Another embodiment of the cement compositions of the present inventioncomprises a hydraulic cement; water; rubber particles wherein the rubberparticles have a mean length of about ¼ inch or less; and carbon fibers.

Another embodiment of the cement compositions of the present inventioncomprises a hydraulic cement wherein the hydraulic cement is a calciumphosphate cement; water; rubber particles; and carbon fibers.

Optionally, other additives suitable for cement compositions such asdispersants, retardants, accelerants, weighting agents, and the like maybe added to the cement compositions of the present invention.

The objects, features and advantages of the present invention will bereadily apparent to those skilled in the art upon a reading of thedescription of the preferred embodiments which follows.

DESCRIPTION

The present invention provides cement compositions having improvedmechanical properties, including tensile strength and elasticity, andassociated methods.

While the compositions and methods are useful in a variety of wellcompletion and remedial operations, they are particularly useful inprimary cementing, e.g., cementing casings and liners in well bores,including those in multi-lateral subterranean wells.

The improved cement compositions of the present invention generallycomprise a hydraulic cement, carbon fibers, rubber particles, and watersufficient to form a pumpable slurry. Other additives suitable for usein subterranean well bore cementing operations also may be added tothese compositions if desired.

Any cement suitable for use in subterranean well cementing operationsmay be used in accordance with the present invention. Preferably, in oneembodiment, the improved cement compositions of the present inventioncomprise a hydraulic cement. A variety of hydraulic cements are suitablefor use in the compositions and methods of the present inventionincluding those comprised of calcium, aluminum, silicon, oxygen, and/orsulfur, which set and harden by reaction with water. Such hydrauliccements include, but are not limited to, Portland cements, pozzolanacements, gypsum cements, high alumina content cements, silica cements,and high alkalinity cements. A preferred cement is commerciallyavailable under the trade designation “THERMALOCK” available fromHalliburton Energy Services, Inc. in Houston, Tex., which is a calciumphosphate cement.

The water utilized in the cement compositions of this invention can befresh water, salt water (e.g., water containing one or more saltsdissolved therein), brine (e.g., saturated salt water produced fromsubterranean formations), or seawater. Generally, the water can be fromany source provided that it does not contain an excess of compounds thatadversely affect other components in the cement composition. The watermay be present in an amount sufficient to form a pumpable slurry. Moreparticularly, the water is present in the cement compositions in anamount in the range of from about 25% to about 100% by weight of cementtherein, more preferably in the range of from about 30% to about 50% byweight of cement material therein.

The carbon fibers that are present in the cement compositions of thepresent invention preferably have a mean length of about 1 mm or less.In certain preferred embodiments, the mean length of the carbon fibersis from about 50 to about 500 microns. Most preferably, the fibers havea mean length in the range of about 100 to about 200 microns.Preferably, they are milled carbon fibers. An example of suitable carbonfibers includes the commercially available “AGM-94” carbon fibersavailable from Asbury Graphite Mills Inc. that have a mean length ofabout 150 microns and a diameter of about 7.2 microns. Preferably, thecarbon fibers are present in the amount of about 1% by weight of cementto about 15% by weight of cement in the cement composition.

The rubber particles that may be used in the cement compositions of thepresent invention may be ¼ inch or less, preferably in the range ofabout 10/20 to 20/30 mesh. The particles can be obtained from anysuitable source. One example of such a suitable source is recycledautomobile tires, which may be obtained from, for example, Four DCorporation, Duncan, Okla. Vulcanized rubber particles are suitable.Preferably, the rubber particles are present in the amount of about 5%to about 50% by weight of the cement in the cement composition. The mostpreferred range is from about 10% to about 40% by weight of cement inthe composition.

It has been found that adding rubber particles to a cement compositionaffects the mechanical properties of the cement composition by, interalia, improving its elasticity and ductility. This is desirable tocounteract the possible stresses the cement may endure. However, whenrubber particles are added in quantities sufficient to desirably affectthe elasticity of the cement, the tensile strength of the cement is alsoreduced. The risk of rupture of the cement sheath in response to astressful condition is directly linked to the tensile strength of thecement, and the risk is attenuated when the ratio of the tensilestrength of the cement to its Young's Modulus is increased. Thus, addingcarbon fibers to a cement composition that comprises rubber particles isdesirable to increase the tensile strength of the cement compositioncontaining the rubber particles. Also, adding carbon fibers as opposedto other additives, such as polypropylene, has the added benefit ofproviding increased temperature stability to the cement composition.This makes the cement compositions of the present invention especiallysuitable for use in or in conjunction with hostile well bore conditions,such as high temperatures and/or high pressures.

As will be recognized by those skilled in the art, when the cementcompositions of the present invention are utilized for primary orremedial subterranean well operation, such compositions can also includeadditional suitable additives, for example, dispersants, fluid lossagents, weighting materials, and the like. While a variety ofdispersants known to those skilled in the art may be used in accordancewith the present invention, a preferred dispersing agent is awater-soluble polymer prepared by the caustic catalyzed condensation offormaldehyde with acetone wherein the polymer contains sodium sulfategroups. Such a preferred dispersing agent is commercially availableunder the trade designation “CFR-3” from Halliburton Energy Services ofDuncan, Okla. Another suitable dispersant is commercially availableunder the trade designation “CFR-2” from Halliburton Energy Services inDuncan, Okla.

The cement compositions of the present invention also can include otheradditives such as accelerants or retarders, if desired. If an accelerantis used, the accelerant is preferably calcium chloride and is present inan amount in the range from about 1.0% to about 2.0% by weight of thecement in the compositions. Fluid loss additives such ashydroxyethylcellulose, carboxymethylcellulose,carboxymethylhydroxyethylcellulose, hydroxypropylcellulose,hydroxypropylguar, guar, polyvinylalcohol, or polyvinylacetate are alsosuitable.

An example of a preferred cement composition of the present invention isa composition of THERMALOCK calcium phosphate cement, sufficient waterto form a pumpable slurry, 1% by weight of the cement CFR-3 dispersant,20% by weight of the cement 20/30 mesh rubber particles, and 5% byweight of the cement milled carbon fibers having a mean length of 150microns.

A preferred method of the present invention comprises providing a cementcomposition that comprises rubber particles and carbon fibers;introducing this cement composition to a subterranean well bore; andallowing the cement composition to set therein.

To facilitate a better understanding of the present invention, thefollowing examples of some of the preferred embodiments are given. In noway should such examples be read to limit the scope of the invention.

EXAMPLES

Test samples of preferred embodiments of the cement compositions of thepresent invention were made and the tensile strength of each compositionwas determined. Comparative samples were also made and similarly tested.To prepare the samples of the cement compositions, THERMALOCK calciumphosphate cement was mixed with 35% by weight of the cement water, andcured for 14 days at 600° F. To certain sample cement composition,rubber particles and/or carbon fibers were added in chosen ratios asdescribed in Table 1. The rubber particles were 20/30 mesh rubberparticles derived from recycled automobile tires and obtained from FourD Corporation of Duncan, Okla. The carbon fibers were milled fibers,specifically AGM-94 fibers from Asbury Graphite Mills Inc., with a meanlength of 150 microns and a diameter of 7.2 microns. The tensilestrength of each cement composition was then determined. All tests wereperformed in accordance with ASTM C190-85.

Table 1 below lists the percentage of rubber particles and carbon fibersthat were added to each cement composition and the resultant tensilestrength. TABLE 1 20/30 Mesh Rubber Milled Carbon Particles FibersTensile Sample (% by weight of (% by weight of Strength Descriptioncement) cement) (psi) Comparative 0 0 480 Sample No. 1 Comparative 0 5510 Sample No. 2 Comparative 20 0 60 Sample No. 3 Example of 20 5 300Preferred Embodiment Sample No. 4

Comparative Sample No. 1 illustrates the tensile strength of a cementcomposition when neither rubber particles nor carbon fibers have beenadded to the composition. This was used as a control sample. The tensilestrength was 480 psi.

Comparative Sample No. 2 illustrates the tensile strength of a cementcomposition containing milled carbon fibers but not rubber particles.The tensile strength was 510 psi, increased from Comparative Sample No.1.

Comparative Sample No. 3 illustrates the tensile strength of a cementcomposition comprising rubber particles. As is evident from Table 1, theaddition of 20/30 mesh rubber particles to the cement compositionreduces the tensile strength of the cement. The tensile strength was 60psi.

As can be seen from Sample 4, which is a typical composition of thepresent invention, by adding milled carbon fibers to the cementcomposition comprising mesh rubber particles, the tensile strength ofthe cement comprising the mesh rubber particles was increased by afactor of 5 to 300 psi; thus, indicating that the tensile strength ofthe cement was enhanced by the addition of the carbon fibers.

Therefore, the present invention is well-adapted to carry out theobjects and attain the ends and advantages mentioned as well as thosewhich are inherent therein. While numerous changes may be made by thoseskilled in the art, such changes are encompassed within the spirit ofthis invention as defined by the appended claims.

1. A cement composition comprising: a hydraulic cement; water; rubberparticles; and carbon fibers wherein the carbon fibers have a meanlength of about 1 mm or less.
 2. The cement composition of claim 1wherein the carbon fibers have a mean length of about 50 microns toabout 500 microns.
 3. The cement composition of claim 1 wherein thecarbon fibers have a mean length of about 100 microns to about 200microns.
 4. The cement composition of claim 1 wherein the carbon fibersare present in an amount of about 1% to about 15% by weight of a cementcomponent of the cement composition.
 5. The cement composition of claim1 wherein the rubber particles have a mean length of about ¼ inch orless.
 6. The cement composition of claim 1 wherein the rubber particlesare present in an amount of about 5% to about 50% by weight of a cementcomponent of the cement composition.
 7. The cement composition of claim1 wherein the hydraulic cement is a Portland cement, a pozzolanacements, a gypsum cement, a high alumina content cement, a silicacement, a high alkalinity cement, a calcium phosphate cements, or amixture thereof.
 8. The cement composition of claim 1 further comprisingan additive wherein the additive is a dispersant, a fluid loss agent,weighting materials, an accelerant, a retardant, or a mixture thereof.9. The cement composition of claim 1 wherein the cement composition hasa tensile strength greater than 60 psi.
 10. A cement compositioncomprising: a hydraulic cement; water; rubber particles wherein therubber particles have a mean length of about ¼ inch or less; and carbonfibers.
 11. The cement composition of claim 10 wherein the rubberparticles are present in an amount of about 5% to about 50% by weight ofa cement component of the cement composition.
 12. The cement compositionof claim 10 wherein the cement composition has a tensile strengthgreater than 60 psi.
 13. A cement composition comprising: a hydrauliccement wherein the hydraulic cement is a calcium phosphate cement;water; rubber particles; and carbon fibers.
 14. The cement compositionof claim 13 wherein the carbon fibers have a mean length of about 1 mmor less.
 15. The cement composition of claim 13 wherein the carbonfibers have a mean length of about 50 microns to about 500 microns. 16.The cement composition of claim 13 wherein the carbon fibers have a meanlength of about 100 microns to about 200 microns.
 17. The cementcomposition of claim 13 wherein the carbon fibers are present in anamount of about 1% to about 15% by weight of a cement component of thecement composition.
 18. The cement composition of claim 13 wherein therubber particles have a mean length of about ¼ inch or less.
 19. Thecement composition of claim 13 wherein the rubber particles are presentin an amount of about 5% to about 50% by weight of a cement component ofthe cement composition.
 20. The cement composition of claim 13 whereinthe cement composition has a tensile strength greater than 60 psi.